1. Field of the Invention
The present invention relates to a radiation detector provided with a photoelectric converting layer in which incident radiation is converted into charges.
2. Description of the Related Art
An active-matrix type plane detector largely attracts attention as an image detector for X-ray diagnosis of new generation. In the plane detector, an X-ray photographed image or a real time X-ray image is outputted as digital signals by detecting irradiated X-rays Moreover, the plane detector has an extremely great expectation even in terms of image quality performance and stability since it is a solid detector.
As a first usage of practical use, the plane detector has been developed for chest radiography or general radiography collecting a still image under a comparatively high dose, and commercialized in recent years. In the near future, commercialization is also expected for application in fields of circulatory organ and digestive organ where it is necessary to realize a real time moving image of not less than 30frames per second with higher performance and under through-view dose. For the usage of the moving image, improvement of S/N ratio and a real time processing technology or the like of minute signals become important development items.
Then, as for this kind of the plane detector, there are two kinds of methods of a direct method and an indirect method broadly classified. The direct method is a method in which photoconductive charges that are generated inside the body are directly converted into signal charges at the inner part of an X-ray photoconductor layer of an a-Se or the like by a high electric field, and these converted signal charges are accumulated in a capacitor for charge accumulation. Moreover, in the direct method, resolution characteristic which is nearly prescribed by a pixel pitch of the active matrix can be obtained, since the photoconductive charges generated by the incident X-ray are directly led to the capacitor for charge accumulation by the high electric field.
On the other hand, in the indirect method, after the incident X-ray is received by a scintillator layer and once it is converted into the visible light, the visible light is led to the capacitor for the charge accumulation by converting the light into the signal charges by an a-Si photodiode or a CCD. Moreover, in the indirect method, the resolution is deteriorated in accordance with optical diffusion and scattering until the visible light from the scintillator layer reaches the photodiode.
Further, as for the X-ray detector of the direct method, a TFT circuit board as a photoelectric converting part at which the capacitor for the accumulation, a thin film transistor (TFT), and a pixel electrode are respectively installed is provided at every pixel arranged on a substrate in a matrix shape. Then, for example, as shown in Japanese Patent Application Publication (KOKAI) No. 2003-209238(page 3-7, FIG. 2), a constitution in which X-ray photoconductive films are laminated on a flattening layer including the pixel electrode of the TFT circuit board as the photoconductive layer is known.
Namely, in this kind of the X-ray detector of the direct method, an “X-ray photoconductive material” is necessary in which the incident X-ray is directly converted into the charge signals. Then, these X-ray photoconductive materials are made as a kind of semiconductors. Further, as the main usage of an image detector which is an X-ray detector of the direct method, the size just sufficient to be able to cover a human body is necessary since there are many cases of using information from the X-rays made being permeated through the human body for medical use. For these reasons, the X-ray detector having about 40 cm length on one side is frequently used as an ordinarily used size.
At this time, if the X-ray detector of the direct method is attempted to be realized, the X-ray photoconductive films must be formed uniformly on the TFT circuit board having the size above 40 cm. Moreover, in order to fully detect the incident X-ray, the X-ray photoconductive films having the thickness of several hundreds μm are necessary even when using materials which are constituted of heavy metals and which have a large specific density. Namely, it is necessary to form a kind of semiconductor film having the size of even 40 cm of length on one side.
Further, as the X-ray detector of the direct method, amorphous selenium (a-Se) is used as the X-ray photoconductive material of the X-ray photoconductive film. However, when using the a-Se as the X-ray photoconductive material, a film-thickness of the X-ray photoconductive film of the a-Se is formed to be around 1 mm, and for example the a-Se is used as the X-ray photoconductive material by applying a strong bias electric field of around 10 V/μm to both ends of the X-ray photoconductive film in order to increase photoconductive charge forming rate per one piece of X-ray photon, in order to make the formed photoconductive charges reach the pixel electrode without being trapped of the formed photoconductive charges by a defect level in the film, and in order to suppress the bias electric field and the diffusion of the charges in the direction at right angles to the bias electric field.
Moreover, as the X-ray detector of other direct methods, a heavy metal compound such as lead iodide (PbI2) and mercury iodide (HgI2) is used as the X-ray photoconductive material. In this case, a heavy metal compound such as these lead iodide (PbI2) and mercury iodide (HgI2) is formed on the TFT circuit board in a film-like shape by vacuum vapor-deposition, or a solution is formed by mixing powders of these heavy metal compounds into a resin solution having charge transferability, and by drying the solution after the solution is coated on the TFT circuit board, the X-ray photoconductive film is made.
However, when using the a-Se as the X-ray photoconductive material as described above, there is a problem in stability since detection sensitivity for converting the X-rays by the a-Se into the charges is low, and a recrystallization temperature is low. Further, in the a-Se, the film thickness of around 1 mmt is necessary since X-ray absorption rate is low due to a small atomic number. Further, manufacturing cost becomes increasing since material efficiency is not so superior for forming the a-Se of the film thickness of around 1 mmt by a vapor-deposition method which is a fabricating method of the a-Se.
Moreover, in composite coating formation with a highly sensitive photoconductive material and an organic material binder, which is another fabricating method when using the a-Se as ray photoconductive material, it is not easy to fully obtain original characteristic of the highly sensitive photoconductive material since the a-Se is a composite with the organic material binder. Further, when the highly sensitive photoconductive material is formed by the vapor-deposition, this formation has problems that unnecessary fabricating cost is charged since the material efficiency is not so superior, and it is not easy to obtain the X-ray photoconductive film of which characteristic is superior.